In oil and gas wells, downhole pumps are used to lift fluid to the surface. One type of downhole pump is called a progressive cavity pump (“PCP”), which is a screw type pump wherein a screw shaped rotor turns inside a stationary stator. The rotor is typically made from steel and the stator is typically a rubber lined steel tube. When the rotor turns, the fluid flows between the rotor and the stator and is forced up the tubing.
The PCP is driven from surface by a rod or rods. The rod is driven by a motor and gearbox called a head unit. The motor is typically electric and powered by a variable frequency drive (“VFD”). The rotational speed of the motor and pump is controlled by changing the frequency of the VFD, which changes the rate of fluid production from the well.
As the rotational speed and load on the pump changes, the rods shorten or lengthen, causing the position of the rotor to move up or down in the stator. For optimum performance of a PCP, the rotor should be vertically centered in the stator and should not protrude from the top or bottom of the stator. The rotational speed at the downhole rotor can vary cyclically or intermittently even with constant rotational speed at the head unit. The rotor may stop and start, move up and down, windup and release etc., which damages the pump and rods, shortens the life of the pump, and reduces pumping efficiency.
Pressure, temperature, and acceleration are routinely measured in wells and monitoring the rods in a downhole pump has been attempted before. Related art includes U.S. Pat. Nos. 4,968,934; 5,666,050; 8,487,626; U.S. Pat. Application Publication No. 2006/0000605, and U.S. Patent Application Publication No. 2015/0060141.